Computer memories may be conveniently classified in terms of whether or not the memory retains the information stored therein when power is removed from the memory. Conventional DRAMs and SRAMs are examples of memories that lose their contents when power is removed. EEPROM and flash RAM are examples of non-volatile memories. The cost of non-volatile memories per bit remains sufficiently high to discourage their use in many applications. In addition, the underlying memory structures may only be written a relatively small number of times compared to volatile memories. For example, an EEPROM memory cell can only be written approximately 10.sup.4 times. In addition, the time required to write data into an EEPROM is much longer than that required to write volatile memories. Hence, EEPROM cells have a relatively limited class of applications.
One class of non-volatile memory that holds the potential for providing increased write cycles and faster writes is based on ferroelectric capacitors. These capacitors have a ferroelectric dielectric which may be polarized in one of two directions. The direction of polarization is used to store information, a "1" corresponding to one direction of polarization and a "0" corresponding to the other direction of polarization. The polarization of the dielectric is maintained when power is removed from the system, thus providing non-volatile operation.
The direction of the polarization may be sensed by applying a potential sufficient to switch the polarization across the capacitor. For the purposes of this discussion, assume that the applied potential difference is such that it would switch the dielectric to the polarization state corresponding to a "1". If the capacitor was polarized such that it stored a "1" prior to the application of the read potential, the polarization will not be altered by the read voltage. However, if the capacitor was polarized such that it stored a "0" prior to the application of the read potential, the polarization direction will switch. This switching will give rise to a current that flows from one plate of the capacitor to the other. A sense amplifier measures the current that flows in response to the read potential to determine the state of the capacitor. Once the capacitor has been read, the data must be rewritten in the capacitor if the read potential caused he state of the capacitor to switch.
A ferroelectric capacitor is normally constructed by depositing a layer of the ferroelectric material on a bottom electrode and then depositing a top electrode on the ferroelectric layer. Ferroelectric layers based on PZT are well known to those skilled in the art. These materials are heated to relatively high temperatures after deposition to provide a perovskite structure having the desired ferroelectric properties. After the annealing process, the dielectric film consists of a large number of domains. Each individual domain has a spontaneous polarization equivalent to that of a mono-domain single crystal of the perovskite material. At the end of the deposition process, domains are randomly oriented.
While this type of memory has been known to the art for some time, commercial realizations of this type of memory have been limited because of two problems, commonly referred to as "imprint" and "fatigue". Imprint is the tendency of a ferroelectric capacitor to exhibit a shift of its hysteresis curve along the voltage axis in either the positive or negative direction depending on the data stored therein. This tendency can lead to a logic state failure for either of two reasons. First, after a sufficient shift, both logic states appear the same to a sense amplifier. Second, the coercive voltage becomes too large to be switched by the available programming voltage. When either case is encountered, a memory cell based on the capacitor becomes useless.
Fatigue is decrease in the magnitude of the remanent polarization of the dielectric layer with the number of times the direction of polarization is changed. Since the amount of charge displaced when the capacitor is switched is related to the remanent polarization, the capacitor finally reaches a point at which there is insufficient charge displaced to detect. At this point, a memory cell based on the capacitor also becomes useless.
Broadly, it is the object of the present invention to provide an improved ferroelectric capacitor for use in memory devices and the like.
It is a further object of the present invention to provide a ferroelectric capacitor that exhibits reduced imprint compared to prior art ferroelectric capacitors.
It is yet another object of the present invention to provide a ferroelectric capacitor that exhibits less fatigue than prior art ferroelectric capacitors.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.